1. Field of the Invention
The present invention relates to a golf club head in which a head main body and a surface material are integrated by bonding together and a manufacturing method thereof. Meanwhile, the “head main body” mentioned in this specification and the scope of claim for a patent refers to a section which to be is integrated with a golf shaft and has a face section, the head main body being formed integrally of metallic material or fiber reinforced resin material. The “surface material” refers to a member which forms the surface of a section in which the golf club head is located while it is integrated with this head main body, this surface material constructing a crown part or a back side face and being formed of metallic material or fiber reinforced resin material.
2. Description of the Related Art
Conventionally, golf club heads in which the head main body and the surface material are integrated by bonding together and a manufacturing method of such golf club head have been proposed.
For example, according to a patent document 1, in a golf club head comprised of a club face, a crown section, a sole section, a rear section, a neck section and the like, the club face, the central portion of the sole section, the central portion of the rear section and both portions of the crown section are formed of a single metallic reinforced plate as the head main body, while the surface material is formed of carbon fiber reinforced composite resin material for the other sections, and both the members are bonded together along their border peripheries with adhesive agent.
According to the patent document 2, a main body is cast with a mold in which a strike face plate is provided and a shaft fixing base is extended from an end of this strike face plate while a concave section is formed on an opposite side to the strike face of the strike face plate. After the angle of the shaft fixing base is adjusted by trimming, an engagement hole is made by tapping a bottom section of the main body. Next, the main body is subjected to heat treatment and polishing treatment. The periphery of the concave section in the main body is sand-blasted with alumina into rough phase. After that, a cover section made of carbon fiber reinforcing agent of multiple layers and having a hole for weight is joined with the concave section and a weight is engaged with the hole for weight so as to obtain a golf club head. Then, this golf club head is placed within a mold and an air bag is taken into the golf club head through the engagement hole and the air bag is filled with gas until the cover section makes a contact with a cavity in the mold and the golf club head is left to harden by hot pressing. After that, the air bag is taken out. Then, a filler block is engaged with the engagement hole.
Further, this inventor has proposed through Japanese Patent No.2002-244725, a manufacturing method of golf club head which comprises a hosel section, a face section, a crown section, a side section and a sole section while a head shell hallow inside is comprised of a first outer shell made of metallic material and a second outer shell made of fiber reinforced resin material, the manufacturing method comprising: a step of forming the outer shell of metallic material having the hosel section, face section and sole section, a step of obtaining an outer shell preparatory formed body by attaching a prepreg sheet in which the reinforced fibers are impregnated with thermosetting resin, a step of overlaying the prepregs in which the reinforced fibers are impregnated with thermosetting resin at positions corresponding to the crown section, side section and sole section of a head forming core material and taking out that core material so as to obtain the outer shell main body preparatory formed body comprised of the crown section, side section and sole section, a step of obtaining a head preparatory formed body by combining the outer shell preparatory formed body with the outer shell main body preparatory formed body and disposing a tube member within a hallow section in the head preparatory formed body through a hole in the hosel section and a step of placing the head preparatory formed body in a mold, closing the mold and heating the head preparatory formed body in the mold by filling the tube member with gas and applying pressure so as to obtain a head formed body.
As disclosed in the above-mentioned conventional arts, generally, to join the head main body and the surface material together, both the members are formed separately and joined together with adhesive agent applied to the joining sections of the both members or a preliminarily formed metallic material member is disposed within a golf club head forming mold and resin material for the surface material is hardened and formed integrally with the metallic material member.
After the joining of the both members is completed, burr formed by hardening of excess adhesive agent at the time of joining with the adhesive agent or excess resin at the time of the integral forming is removed and gaps generated in the hardened adhesive agent or resin are filled with putty. After polishing and painting, a golf club head is completed.    Patent document 1: Japanese Utility Model Application Laid-Open No. HEI 7-406    Patent document 2: Japanese Patent Application Laid-Open No. 2001-340499
However the golf club heads produced by joining the head main body and the surface material according to the above-mentioned conventional arts have following problems.
Because the golf club head is used for striking a ball repeatedly, vibration due to impact by the striking is generated entirely in the golf club head each time it hits the ball. Thus, because in a golf club head in which the head main body and the surface material are integrated, the properties of both the members are different, differences in transmission of vibration by such impact and the amount of deformation in material due to the impact are generated. Consequently, deviation or gap is generated in the joining section between the both members, thereby finally leading to destruction of the golf club head, which is a problem to be solved.
Further, in a golf club head constructed by joining the both members together with adhesive agent, the durability can be improved by intensifying the coupling force of the both members by enlarging the bonding area of the joining section or using strong adhesive agent. In a golf club head constructed by integral forming of the both members also, the durability can be improved by intensifying the coupling force of the both members by using a formation use adhesive agent having an excellent fitting performance to metallic material.
However, although in the golf club heads whose durability is improved, the coupling force can be intensified for the improvement of the durability, it is difficult to eliminate completely slight deformation in material and deviation and gap due to vibration on the border between the both members. Further, a crevice is generated in a coating film applied across the border on the joining section accompanied by the deviation or gap which may be generated when a ball is hit repeatedly with the golf club head finally finished by coating. For this reason, customer complaints occur and its specified quality cannot be maintained, which is a problem to be solved.